CSE 311: Foundations of Computing. Lecture 6: More Predicate Logic

Transcription

1 CSE 311: Foundations of Computing Lecture 6: More Predicate Logic

2 Last class: Predicates Predicate A function that returns a truth value, e.g., Cat(x) ::= x is a cat Prime(x) ::= x is prime HasTaken(x, y) ::= student x has taken course y LessThan(x, y) ::= x < y Sum(x, y, z) ::= x + y = z GreaterThan5(x) ::= x > 5 HasNChars(s, n) ::= string s has length n Predicates can have varying numbers of arguments and input types.

3 Last class: Domain of Discourse For ease of use, we define one type / domain that we work over. This set of objects is called the domain of discourse. For each of the following, what might the domain be? (1) x is a cat, x barks, x ruined my couch (2) x is prime, x = 0, x < 0, x is a power of two (3) x is a pre-req for z

4 Domain of Discourse For ease of use, we define one type / domain that we work over. This set of objects is called the domain of discourse. For each of the following, what might the domain be? (1) x is a cat, x barks, x ruined my couch mammals or sentient beings or cats and dogs or (2) x is prime, x = 0, x < 0, x is a power of two numbers or integers or integers greater than 5 or (3) x is a pre-req for z courses

5 Last Class: Quantifiers We use quantifiers to talk about collections of objects. x P(x) P(x) is true for every x in the domain read as for all x, P of x x P(x) There is an x in the domain for which P(x) is true read as there exists x, P of x

6 Last class: Statements with Quantifiers Domain of Discourse Positive Integers Predicate Definitions Even(x) ::= x is even Odd(x) ::= x is odd Prime(x) ::= x is prime Determine the truth values of each of these statements: x Even(x) x Odd(x) x (Even(x) Odd(x)) x (Even(x) Odd(x)) x Greater(x+1, x) x (Even(x) Prime(x)) Greater(x, y) ::= x > y Equal(x, y) ::= x = y Sum(x, y, z) ::= x + y = z

7 Statements with Quantifiers Domain of Discourse Positive Integers Predicate Definitions Even(x) ::= x is even Odd(x) ::= x is odd Prime(x) ::= x is prime Determine the truth values of each of these statements: Greater(x, y) ::= x > y Equal(x, y) ::= x = y Sum(x, y, z) ::= x + y = z x Even(x) x Odd(x) x (Even(x) Odd(x)) x (Even(x) Odd(x)) x Greater(x+1, x) x (Even(x) Prime(x)) T e.g. 2, 4, 6,... F e.g. 2, 4, 6,... T every integer is either even or odd F no integer is both even and odd T adding 1 makes a bigger number T Even(2) is true and Prime(2) is true

8 Statements with Quantifiers Domain of Discourse Positive Integers Predicate Definitions Even(x) ::= x is even Odd(x) ::= x is odd Prime(x) ::= x is prime Translate the following statements to English Greater(x, y) ::= x > y Equal(x, y) ::= x = y Sum(x, y, z) ::= x + y = z x y Greater(y, x) x y Greater(x, y) x y (Greater(y, x) Prime(y)) x (Prime(x) (Equal(x, 2) Odd(x))) x y (Sum(x, 2, y) Prime(x) Prime(y))

9 Statements with Quantifiers (Literal Translations) Domain of Discourse Positive Integers Predicate Definitions Even(x) ::= x is even Odd(x) ::= x is odd Prime(x) ::= x is prime Translate the following statements to English Greater(x, y) ::= x > y Equal(x, y) ::= x = y Sum(x, y, z) ::= x + y = z x y Greater(y, x) For every positive integer x, there is a positive integer y, such that y > x. x y Greater(x, y) For every positive integer x, there is a positive integer y, such that x > y. x y (Greater(y, x) Prime(y)) For every positive integer x, there is a pos. int. y such that y > x and y is prime. x (Prime(x) (Equal(x, 2) Odd(x))) For each positive integer x, if x is prime, then x = 2 or x is odd. x y (Sum(x, 2, y) Prime(x) Prime(y)) There exist positive integers x and y such that x + 2 = y and x and y are prime.

10 Statements with Quantifiers (Natural Translations) Domain of Discourse Positive Integers Predicate Definitions Even(x) ::= x is even Odd(x) ::= x is odd Prime(x) ::= x is prime Translate the following statements to English Greater(x, y) ::= x > y Equal(x, y) ::= x = y Sum(x, y, z) ::= x + y = z x y Greater(y, x) There is no greatest integer. x y Greater(x, y) There is no least integer. x y (Greater(y, x) Prime(y)) For every positive integer there is a larger number that is prime. x (Prime(x) (Equal(x, 2) Odd(x))) Every prime number is either 2 or odd. x y (Sum(x, 2, y) Prime(x) Prime(y)) There exist prime numbers that differ by two.

11 English to Predicate Logic Domain of Discourse Mammals Predicate Definitions Cat(x) ::= x is a cat Red(x) ::= x is red LikesTofu(x) ::= x likes tofu Red cats like tofu Some red cats don t like tofu

12 English to Predicate Logic Domain of Discourse Mammals Predicate Definitions Cat(x) ::= x is a cat Red(x) ::= x is red LikesTofu(x) ::= x likes tofu Red cats like tofu x ((Red(x) Cat(x)) LikesTofu(x)) Some red cats don t like tofu y ((Red(y) Cat(y)) LikesTofu(y))

13 English to Predicate Logic Domain of Discourse Mammals Red cats like tofu When there s no leading quantification, it means for all. Some red cats don t like tofu Predicate Definitions Cat(x) ::= x is a cat Red(x) ::= x is red LikesTofu(x) ::= x likes tofu When putting two predicates together like this, we use an and. When restricting to a smaller domain in a for all we use implication. When restricting to a smaller domain in an exists we use and. Some means there exists.

14 Negations of Quantifiers Predicate Definitions PurpleFruit(x) ::= x is a purple fruit (*) x PurpleFruit(x) ( All fruits are purple ) What is the negation of (*)? (a) there exists a purple fruit (b) there exists a non-purple fruit (c) all fruits are not purple Try your intuition! Which one feels right? Key Idea: In every domain, exactly one of a statement and its negation should be true.

15 Negations of Quantifiers Predicate Definitions PurpleFruit(x) ::= x is a purple fruit (*) x PurpleFruit(x) ( All fruits are purple ) What is the negation of (*)? (a) there exists a purple fruit (b) there exists a non-purple fruit (c) all fruits are not purple Key Idea: In every domain, exactly one of a statement and its negation should be true. Domain of Discourse {plum} Domain of Discourse {apple} Domain of Discourse {plum, apple} (*), (a) (b), (c) (a), (b)

16 Negations of Quantifiers Predicate Definitions PurpleFruit(x) ::= x is a purple fruit (*) x PurpleFruit(x) ( All fruits are purple ) What is the negation of (*)? (a) there exists a purple fruit (b) there exists a non-purple fruit (c) all fruits are not purple Key Idea: In every domain, exactly one of a statement and its negation should be true. Domain of Discourse {plum} Domain of Discourse {apple} Domain of Discourse {plum, apple} (*), (a) (b), (c) (a), (b) The only choice that ensures exactly one of the statement and its negation is (b).

17 De Morgan s Laws for Quantifiers x P(x) x P(x) x P(x) x P(x)

18 De Morgan s Laws for Quantifiers x P(x) x P(x) x P(x) x P(x) There is no largest integer x y ( x y) x y ( x y) x y ( x y) x y (y > x) For every integer there is a larger integer

19 Scope of Quantifiers x (P(x) Q(x)) vs. x P(x) x Q(x)

20 scope of quantifiers x (P(x) Q(x)) vs. xp(x) xq(x) This one asserts P and Q of the same x. This one asserts P and Q of potentially different x s.

21 Scope of Quantifiers Example: NotLargest(x) y Greater (y, x) z Greater (z, x) truth value: doesn t depend on y or z bound variables does depend on x free variable quantifiers only act on free variables of the formula they quantify x( y(p(x,y) xq(y, x)))

22 Quantifier Style x( y(p(x,y) xq(y, x))) This isn t wrong, it s just horrible style. Don t confuse your reader by using the same variable multiple times there are a lot of letters

23 Nested Quantifiers Bound variable names don t matter x y P(x, y) a b P(a, b) Positions of quantifiers can sometimes change x (Q(x) yp(x, y)) x y(q(x) P(x, y)) But: order is important...

24 Quantifier Order Can Matter Domain of Discourse Integers OR {1, 2, 3, 4} Predicate Definitions GreaterEq(x, y) ::= x y There is a number greater than or equal to all numbers. x y GreaterEq(x, y))) Every number has a number greater than or equal to it. y x GreaterEq(x, y))) x y T F F F T T F F T T T F T T T T The purple statement requires an entire row to be true. The red statement requires one entry in each column to be true.

25 Quantification with Two Variables expression when true when false x y P(x, y) Every pair is true. At least one pair is false. x y P(x, y) At least one pair is true. All pairs are false. x y P(x, y) Wecan find a specific y for each x. (x 1, y 1 ), (x 2, y 2 ), (x 3, y 3 ) y x P(x, y) Wecan find ONE y that works no matter what x is. (x 1, y), (x 2, y), (x 3, y) Some x doesn t have a corresponding y. For any candidate y, there is an x that it doesn t work for.

26 Logical Inference So far we ve considered: How to understand and express things using propositional and predicate logic How to compute using Boolean (propositional) logic How to show that different ways of expressing or computing them are equivalent to each other Logic also has methods that let us infer implied properties from ones that we know Equivalence is a small part of this

27 Applications of Logical Inference Software Engineering Express desired properties of program as set of logical constraints Use inference rules to show that program implies that those constraints are satisfied Artificial Intelligence Automated reasoning Algorithm design and analysis e.g., Correctness, Loop invariants. Logic Programming, e.g. Prolog Express desired outcome as set of constraints Automatically apply logic inference to derive solution

28 Proofs Start with hypotheses and facts Use rules of inference to extend set of facts Result is proved when it is included in the set

29 An inference rule: Modus Ponens If p and p q are both true then q must be true Write this rule as p, p q q Given: If it is Monday then you have a 311 class today. It is Monday. Therefore, by Modus Ponens: You have a 311 class today.

30 My First Proof! Show that r follows from p, p q, and q r 1. p Given 2. p q Given 3. q r Given 4. 5.

31 My First Proof! Show that r follows from p, p q, and q r 1. p Given 2. p q Given 3. q r Given 4. q MP: 1, 2 5. r MP: 3, 4

32 Proofs can use equivalences too Show that p follows from p q and q 1. p q Given 2. q Given 3. q p Contrapositive: 1 4. p MP: 2, 3

33 Inference Rules Each inference rule is written as:...which means that if both A and B are true then you can infer C and you can infer D. For rule to be correct (A B) C and (A B) D must be a tautologies A, B C,D Sometimes rules don t need anything to start with. These rules are called axioms: e.g. Excluded Middle Axiom p p

34 Simple Propositional Inference Rules Excluded middle plus two inference rules per binary connective, one to eliminate it and one to introduce it p q p, q p q, p q p, p q q p, q p q p x p q, q p p q p q Direct Proof Rule Not like other rules

35 Proofs Show that r follows from p, p q and (p q) r How To Start: We have givens, find the ones that go together and use them. Now, treat new things as givens, and repeat. p, p q q p q p, q p, q p q

36 Proofs Show that follows from,, and Two visuals of the same proof. We will use the top one, but if the bottom one helps you think about it, that s great! MP Intro MP 1. Given 2. Given 3. MP: 1, 2 4. Intro : 1, 3 5. Given 6. MP: 4, 5

37 Important: Applications of Inference Rules You can use equivalences to make substitutions of any sub-formula. Inference rules only can be applied to whole formulas (not correct otherwise). e.g. 1. p q given 2. (p r) q intro from 1. Does not follow! e.g. p=f, q=f, r=t